1. Field of the Invention
This invention relates generally to apparatus for sparging gas into liquid for gas-liquid contacting and bulk mixing thereof.
2. Description of the Prior Art
In carrying out mass-transfer processes between gases and liquids, it is common practice to introduce the gas into the liquid in the form of small bubbles. In such manner a large aggregate interfacial surface area between the gas and liquid may be generated to provide correspondingly high rates of mass transfer.
When the volume of gas to be introduced into the liquid is comparatively small in relation to the liquid volume, sparging systems are frequently employed. The sparger is typically fabricated with small apertures through which gas in injected into the liquid, to provide a relatively fine dispersion of gas bubbles in the liquid undergoing treatment. In such systems, the sparger is commonly positioned at the bottom of a tank so that the small gas bubbles rise slowly through the liquid to provide an extended period of gas-liquid contacting.
When a submerged sparger is employed for gas-liquid contacting in a large body of liquid, it is generally desirable to employ means for bulk mixing of the liquid in the tank. Bulk mixing is utilized to eliminate the conditions under which stagnant liquid zones would otherwise form and to insure that a high mass-transfer gradient is maintained between the gas and liquid and that liquid is recirculated to the location at which the gas is introduced. Bulk mixing is particularly important in applications involving liquids containing solids or particulate matter which must be retained in suspension during the treatment step. Examples of such applications include fermentation, flocculation and activated sludge wastewater treatment.
Thus, in submerged sparger systems the energy required to operate the system generally comprises a portion which is extended to introduce the gas into the liquid in the form of small bubbles and a portion which acts to create bulk circulation in the liquid volume being treated. Accordingly, the efficient use of the energy required for bulk mixing is of prime importance and equipment which employs such energy for the dual purpose of improving gas-liquid contact as well as bulk mixing are favored by reason of their relatively high efficiency. Unfortunately, the prior art systems fail to efficiently integrate the gas injection and bulk circulation functions, and thereby fail to exploit the expended energy in a fashion which is mutually beneficial to both purposes. In other words, these systems fail to realize a significant augmentation to the effect of the one mechanism by the action of the other mechanism.
In processes involving interphase mass transfer, the resistance to such transfer is imposed by the penetration fluid films which exist at the interface between the respective phases and the magnitude of the resistance is a function of the depth of such films. In liquid-gas systems, it is well known to these film depths and the associated mass transfer resistance can be decreased by creating turbulent shearing stresses in the fluid film interfacial region. It is also known that such shearing stresses can increase the interfacial area available for mass transfer by reducing the size of the gas bubbles in the turbulent liquid and by increasing the rate of renewal of the interfacial surface. The shearing stresses can be viewed as creating shear planes over which a slipping or sliding occurs between contiguous layers of fluid, and as a consequence, steep flow velocity gradients exist through the depth of the fluid film interface. The thickness of the interfacial fluid film is directly related to the velocity gradient developed by the turbulent shearing stresses. Despite the knowledge of such mass-transfer enhancement mechanisms, it has not been possible to achieve high energy utilization when such mechanisms are employed because of the turbulent energy dissipation and decay associated therewith which cause a rapid degradation of the energy resident in the respective fluid flows.
The simplest submerged sparger systems involve introduction of the gas into the liquid medium by passage of the gas through a stationary aperture. The size of the resulting formed bubbles are a function of the dimensions of the aperture, the shear stress (which in turn is related to the aperture size, gas flow rate, fluid viscosities and densities) and the interfacial surface tension. Because the surface area to volume ratio of the gas bubbles increases with decreasing bubble diameter, the stationary spargers are usually fabricated with very small apertures in order to promote the formation of small bubbles. The introduction of gas through a porous ceramic medium is a common industrial practice. In such systems, energy must be expended first to overcome the pressure drop created by the flow of gas through the aperture and second to form a gas bubble, which formation requires the expenditure of an amount of work to increase the liquid/gas contact area within the enveloping body of liquid. However, the stationary sparger system is not designed to further enhance mass transfer either at the point of bubble formation or throughout the bulk-liquid volume. In the absence of a significant "gas lift" of the liquid by the gas, the work is expended for the sole purpose of creating surface area over which mass transfer can occur and the sparger is not particularly efficient in its use of energy expended.
Recognizing the inherent inefficiency of simply bubbling gas into a liquid, the prior art has proposed a sparging system in which gas is introduced into a body of liquid through a "bubble dispenser" disposed in a confined flow passage and the liquid is circulated downwardly through the confined flow passage to increase the retention time of the bubbles in the liquid, with the gas bubbles being displaced from the flow passage by crowding at the outlet end thereof.
In addition to increasing the retention time, the fresh supply of circulated liquid to the bubble-emitting "disperser" in the flow passage maintains a high concentration gradient through the interfacial fluid films. Despite these advantages, the resistance to mass transfer imposed by the interfacial fluid films is not materially affected by the downwardly flowing water and in this respect, the apparatus is comparable to a rudimentary stationary sparger.
Various sparger designs have also been proposed by the prior art in which gas bubbles are injected into a flowing system of liquid which is conveyed to a shearing zone, as for example in the vicinity of a high shear propeller. In such zone the bubble size is reduced as larger bubbles are sheared to form smaller bubbles, and the liquid may be given an appreciable downward or radial velocity serving to increase the bubble residence time in the liquid. The shearing action in this zone also serves to decrease the interfacial film resistance. Such type of sparging system is able to achieve comparatively high rates of mass transfer due to the foregoing effects, but does not employ input energy most efficiently. This is because the three process steps of creating bubble surface area, providing a liquid stream to carry the bubbles from the bubble formation zone to the shear zone, and shearing the so-conveyed bubbles in the shearing zone are each separately and independently performed, so that this combination involves little or no augmentation of the effect of the one mechanism by the action of the other mechanisms.
In general, the prior art sparging systems either do not provide adequate shearing action at the point of gas introduction to reduce the intrafacial film resistance for high mass transfer rates or else do not efficiently utilize the energy supplied to the system for mass transfer at the point of gas injection or for bulk mixing and gas circulation throughout the body of the liquid.
As indicated above, sparging systems are frequently employed to treat liquids containing suspended solids. When the gas flow openings of the sparger are small in size, the solids may clog the sparger openings, adversely affecting the system performance and requiring periodic shutdown and cleaning of the sparger. Large gas flow openings may ameliorate the problem but tend to produce correspondingly larger gas bubbles which reduce the aggregate interfacial surface area and mass transfer efficiency of the system.
An additional deficiency of many prior art sparging systems relates to the relationship between the power drawn by the sparger system and the gas load which it is required to handle. In systems which encounter a changing process load, the optimal gas feed rate may change accordingly and the apparatus must be capable of accommodating such changes. For example, in sparger systems wherein the gas-liquid dispersion passes through an impeller, as for example for flow directing of the liquid to increase bubble contact time, for bubble shearing or simply for conveying liquid, the power drawn by the impeller is highly sensitive to gas feed rate. Such apparatus characteristically exhibits a significant increase in power draw with a decrease in gas load, and since the power train must satisfy the full range of operating conditions its size is significantly greater than that required to satisfy operation at full gas load.
Accordingly, it is an object of the present invention to provide an improved apparatus for sparging gas into liquid.
It is also an object of the invention to provide a sparging apparatus for promoting gas-liquid contact which is particularly efficient in its utilization of energy, which permits a large amount of gas to be dissolved per unit of energy expended and which maintains a high rate of bulk circulation within the body of liquid being treated.
It is a further object of the invention to provide a submerged sparger which offers a wide operating range in terms of the liquid and gas processing rates.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.